1. Field of the Invention
This invention relates to voltage-controlled oscillators and to phase-locked loops using such oscillators.
2. Background of the Invention
In conventional phase-locked receivers, such as used in data communication systems using Manchester-encoded data signals, the acquisition time for such a receiver to acquire and to lock into a received signal is a predictable function of the receiver's phase-locked loop time constants. These phase-locked loops are usually of the analog type, that is, they use a continuously variable voltage signal for controlling the frequency of the voltage-control oscillator. One particular problem associated with analog phase-locked loop receivers is that phase-lock cannot be obtained if the input signal is precisely out of phase with the receiver local oscillator signal. In applications of phase-locked loops to communication systems, wide loop bandwidths are desirable for acquisition, while much narrower bandwidths are required for tracking. Previously used techniques, such as switching various capacitors into the circuit, to obtain desired bandwidths are not suitable and are often impractical to implement in high-speed data communication systems.
Manchester-encoded communication systems are used to communicate digital information because they permit a received signal to be quickly acquired and tracked. Signals encoded in conformity with a Manchester-encoding scheme are self-clocking, that is, every bit cell of encoded information has a logic-level transition occurring at its midpoint so that a Manchester-encoded signal contains a significant clock frequency component which is separable from the data information. Manchester systems often transmit a preamble prior to actual data information being transmitted so that a receiver can acquire and lock on the input signal and establish a clock reference signal. A system for deriving a clock signal must effectively operate in the presence of noise and jitter.
Many Manchester-encoded data communication systems, such as the Ethernet Local-Area-Network system, are operated intermittently, that is, in stop-and-start applications. Packets of encoded information occur at random intervals, so that there are many time intervals when a signal line is inactive and has only random or impulse noise signals present. A conventional analog phase-locked loop receiver operating under such conditions has difficulty operating because fast acquisition requires wide loop bandwidths while operation with noisy signals requires narrow loop bandwidths, which require a compromise bandwith to be used.
For integrated circuits, accurate time-delays for pulses, that is, delays controllable to a predetermined time, plus or minus a few percent, are not easily obtained. For fixed delays, discrete delay lines have been used.
When relatively long or when variable delays are desired, discrete delay lines have become quite large and difficult to vary. In many digital systems which operate over a range of frequencies, it would be very convenient to have pulse delays which would automatically track with the pulse frequency.
It is an object of the invention to provide an ECL gate stage having a controlled fall time.
It is another object of the invention to provide a circuit which provides a precise time-delay proportional to the frequency of a reference signal.
It is another object of the invention to provide an interruptible voltage-controlled oscillator.
It is another object of the invention to provide a frequency-to-voltage converter utilizing a phase-locked loop.
It is an object of the invention to provide a voltage controlled oscillator which has its frequency separately controlled from its phase.
It is another object of the invention to provide a clock recovery system useful with intermittently sent data packets.
It is another object of the invention to provide a frequency reference signal having its frequency accurately controlled by one voltage-controlled oscillator circuit while its phase is controlled by a second voltage-controlled oscillator.
It is another object of the invention to provide a voltage-controlled oscillator having a wide frequency-tracking-bandwidth but yet a narrow locking-bandwidth.
It is another object of the invention to provide a circuit which provides a decoding clock from a Manchester-encoded data signal.
It is another object of the invention to provide a circuit for discriminating between noise and a desired signal.
It is another object of the invention to provide a circuit for detecting the presence of Manchester-encoded data signal on a signal line.
In accordance with these and other objects of the invention, an ECL gate stage having a controlled fall time is provided which is especially useful in voltage-controlled oscillators and phase-locked loop circuits. A timing capacitor and a current source are coupled to the output of the ECL gate stage. The current source controls the amount of current charging the timing capacitor to thereby control the fall time of the output signal at the gate output terminal. Both inverting and non-inverting transistors are provided for the ECL gate and the non-inverting input transistor is used to disable the ECL gate.
A voltage-controlled oscillator is provided according to the invention which utilizes a number of the ECL gate stages, timing capacitors, and current sources described above arranged in a ring oscillator configuration. A non-inverting input transistor for one of the gates is used to provide interrupted operation of the voltage-controlled oscillator, a feature particularly useful with intermittently operated systems. A voltage-controlled oscillator assembled in accordance with the invention is combined with a phase-detector in a phase-locked loop configuration. The phase-detector compares the voltage-controlled oscillator signal to a reference signal and provides an output voltage proportional to the frequency of the input signal.
A time-delay proportional to a reference frequency is provided for a pulse signal by an ECL gate stage, a timing capacitor, and a current source controlled by a reference voltage from a phase-locked loop, locked to the reference frequency. A time-delay circuit is also useful as part of a pulse-discriminator circuit. A first time-delay circuit receives an inverted input pulse and a second time-delay circuit receives the input pulse. The inverted, time-delayed signal is fed to the clear input of an edge-triggered flip-flop while the other time-delay signal is fed to the clock input of the edge-triggered flip-flop. A Manchester-encoded data signal can be detected when the delays are appropriately set.
A system is also provided for producing a receiver clock reference signal from a received signal. Two phase-locked loops are used. Each has a voltage-controlled oscillator with its output coupled to one input of a respective phase-detector. The first phase-locked loop phase-detector locks to the frequency of the system reference signal. The phase-detector output is utilized as a control voltage for the first phase-locked loop. This control voltage is also used as an additional control voltage in the second phase-locked loop. The second phase-locked loop has its own voltage-controlled oscillator and phase-detector, which receives a noisy or jittery received signal. The output of the second phase-detector is a control voltage for the phase of the second voltage-controlled oscillator. As mentioned above, the second voltage-controlled oscillator also receives another voltage control signal from the first phase-locked loop. Thus, the second voltage-controlled oscillator receives two control signals, one constraining its frequency to be near that of the system reference signal and the second being used to control the phase of the second phase-locked loop to be that of the received signal. The second phase-locked oscillator is interruptible and this two-oscillator system is particularly useful for providing a receiver clock reference signal for a Manchester-encoded data signal.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or maybe learned by practice of the invention. The objects and advantages of the invention will be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.